﻿.box{
  
      width: 320px;
      height: 320px;
      border: 0px;
      position: absolute;
 
     text-align: center;
 background:#dfdfdf;
     left:0;
    right:0;
    top: 0;
    bottom: 0;
    margin: auto;
    /*50%为自身尺寸的一半*/
}